This invention relates to pressure transducers, and particularly to low cost but precision capacitive pressure transducers.
U.S. Pat. No. 4,295,376 is directed to force responsive capacitive transducers employing a configuration in which a diaphragm member is deflected in response to pressure variations which differentially deflect two different capacitor pairs formed between the diaphgram and a reference plate mounted on the diaphragm itself. In this configuration, an inner electrode pair is defined at or adjacent the central axis of the diaphragm and reference plate, and spacer elements positioned outside the radius of the inner central electrode pair hold the reference plate in generally parallel relation to its original position, despite deflection outwardly as the diaphragm becomes increasingly convex. An outer electrode pair formed by convex surfaces on the diaphragm and the facing part of the reference plate thus has a larger spacing as pressure increases, while the inner electrode pair has a smaller spacing. The opposite deviations in spacing between the inner and outer electrode pairs provide a reliable basis for generating substantially linearly varying signals representative of the pressure that is acting to deform the diaphragm.
This type of capacitive transducer has significant advantages over prior art structures, in which the facing elements and surface electrodes are spaced apart by a peripheral seal.
In a different form of capacitive pressure transducer, exemplified by U.S. Pat. No. 4,089,036 (Geronime), the deflectable diaphragm having an electrode surface bears a center post which serves as the support structure for a separate transverse disk on which another electrode surface is mounted, so that deflection of the diaphragm causes a variation in spacing between the electrodes. However, this system is not readily comparable because it is far more expensive to manufacture and much more cumbersome in use, and cannot be adapted to cover an adequately wide range of variations. The structure of previously referenced U.S. Pat. No. 4,295,376 is superior inasmuch as the component parts may be made of ceramic materials on which the electrodes are deposited as thin films, and the spacer structure between the diaphragm and the reference plate may be made of glassy material that can be deposited to precise thicknesses but has substantial strength when vitrified.
Nonetheless, certain problems have been encountered with this type of structure in meeting the constantly increasing demand for performance and versatility encountered in present applications. It is necessary, for high sensitivity in low pressure applications, to have a very thin diaphragm, which may be in the range of as little as 0.002" to 0.010" or more in thickness, but having relatively large radius so that they are very flexible. A molded ceramic part must be subjected to a precision lapping step in order to provide the precise thickness desired for sensor applications, but with a flexible low pressure range diaphragm the forces exerted during lapping cause slight but still significant deflection of the structure, resulting typically in a less flat or crowned diaphragm structure. This means that although the thicknesses of the diaphragms are uniform, the crowning effect is not, so that differential deflection of the inner electrode pair relative to the outer electrode pair is not the same. Hence, special efforts must be made to assure appropriate linearity.
In addition, the spacers on the diaphragm which couple to the reference plate, being at a substantial radius from the center, are subjected to torsion as the diaphragm curves. To alleviate stress problems, the spacers are coupled to the reference plate at small elongated arc segments that are defined between arc apertures in the reference plates, thus permitting torsional effects to be absorbed with lower stress on the spacers.
However, these radially positioned spacers cannot be large in size, and therefore a limit is reached as to the shock and vibration sensitivity of these parts. In addition, it must be borne in mind that the spacings involved are very small, typically being less than a few thousandths of an inch between the electrodes of a pair. A number of problems have arisen because the deflection characteristics of thinner (low pressure range) diaphragms are different than those for the thicker (high pressure range) diaphragms. With differentially deflected capacitor pairs, this means that device geometries have to be changed or that signal compensation networks have to be used. Either approach introduces substantial elements of cost. Thus fabrication problems can be significant, because it is desirable to assemble large quantities of units on a production basis. In addition, the electrode patterns on the different elements require specific geometries, in order that conductors into the different elements can follow non-interfering circuit paths.